Revert "Add stable references of macro_metavar_expr"

Author: tshepang

src/macros-by-example.md

@@ -193,53 +193,6 @@ compiler knows how to expand them properly:
     not have the same number. This requirement applies to every layer of nested
     repetitions.
 
-## Dollar-dollar ($$)
-
-`$$` expands to a single `$`.
-
-Since metavariable expressions always apply during the expansion of a macro, they cannot be used in recursive macro definitions and this is where `$$` expressions comes into play, i.e., `$$` can be used to resolve ambiguities in nested macros.
-
-The following example illustrates a macro that fails to compile due to the ambiguity of the repetition in a nested macro:
-
-```rust,compile_fail
-macro_rules! foo_error {
-    () => {
-        macro_rules! bar_error {
-            ( $( $any:tt )* ) => { $( $any )* };
-            // ^^^^^^^^^^^ error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
-        }
-    };
-}
-
-foo_error!();
-```
-
-The following resolves the problem by escaping the `$` in the repetition with `$$`:
-
-```rust
-macro_rules! foo_ok {
-    () => {
-        macro_rules! bar_ok {
-            ( $$( $any:tt )* ) => { $$( $any )* };
-        }
-    };
-}
-
-foo_ok!();
-```
-
-One consequence of such expansion is that deeper nested levels make dollar-dollar declarations grown linearly, starting at `$$`, then `$$$$`, then `$$$$$` and so on. This is also necessary to be fully featured so that it is possible to specify names of metavariables using other metavariables at each nesting level.
-
-```ignore
-$foo          => bar      => bar    // Evaluate foo at level 1
-$$foo         => $foo     => bar    // Evaluate foo at level 2
-$$$foo        => $bar     => baz    // Evaluate foo at level 1, and use that as a name at level 2
-$$$$foo       => $$foo    => $foo   // Evaluate foo at level 3
-$$$$$foo      => $$bar    => $bar   // Evaluate foo at level 1, and use that as a name at level 3
-$$$$$$foo     => $$$foo   => $bar   // Evaluate foo at level 2, and use that as a name at level 3
-$$$$$$$foo    => $$$bar   => $baz   // Evaluate foo at level 1, use that at level 2, and then use *that* at level 3
-```
-
 ## Scoping, Exporting, and Importing
 
 For historical reasons, the scoping of macros by example does not work entirely